I am considering such issues as climatology and tectonic activity. I like dinosaurs, and I think the possibilities of very big dinosaurs would be good – see Lost World for why I like ‘em. I am thinking a world that is more oxygen rich than ours would give larger animals: insects and reptiles (and birds) included. Greater feats of athleticism are possible with greater oxygenation.
Another possibility is lower gravity, which makes for a greater variety of flying creatures – as well as more heroic leaping and jumping by non-flyers as well. Actually, lower gravity could be caused by less metals in the core (less iron, nickel), which would change economics and warfare – and perhaps delay any industrial revolution tendencies.
So gravity of – say 80% earth normal. Free oxygen about 30% (earth is 21%). Mountains could then be higher, and life could exist further up them than on earth.
More of the planet would be primordial swamp and rain forest both tropical and temperate. For an element of uncertainty increased tectonic activity would be fun – offering natural disasters to wipe out civilisations periodically with Krakatoa like eruptions.
My final decision as to a chief distinguishing feature, is to have the planet knocked off its kilter, and have the polar axes wander across the entire globe. The climate will constantly shift so that every generation, a particular location will swing from one climate zone to the next. From polar complete with sea ice and glaciers, then 30-40 years later, sub-arctic with permafrost and no trees, then another 30-40 years, temperate, then subtropical, then tropical, and back through the zones until it is polar arctic once more. Coupled with at least one pretty large continental mass (which will allow migration to stay with a particular climate zone), and we have something that will encourage a nomadic existence, or at least inhibit “civilization” (city building) significantly.